1. Field of the Invention
This invention relates to an inverted Galilean telescope type viewfinder optical system, and in particular to an inverted Galilean telescope type viewfinder system of the magnification change-over type suitable for use in a focal length change-over type compact camera or an electronic camera or the like.
2. Related Background Art
Heretofore, in a focal length change-over type compact camera or the like, use has often ben made of an inverted Galilean telescope type viewfinder of the type in which the viewfinder magnification is changed over in response to the change-over of the focal length of a photo-taking lens. As such a viewfinder of the magnification change-over type, there are known the techniques disclosed, for example, in U.S. Pat. No. 2,755,701, Japanese Laid-Open Patent Application No. 166933/1985 and Japanese Laid-Open Patent Application No. 26323/1985.
Of these known techniques, the viewfinder disclosed in U.S. Pat. No. 2,755,701 is of the type in which only a negative lens in an objective lens comprising a positive lens and a negative lens is slidden in the direction of the optic axis to thereby change the combined focal length of the objective lens and change the viewfinder magnification, and if the magnification change ratio becomes greater, the range of movement of the negative lens for a magnification change becomes greater, and this has lead to the disadvantage that such viewfinder cannot be made into a compact viewfinder of short full length. Also, there is a system in which mounting and dismounting of an additional negative lens and sliding of another negative lens in the direction of the optic axis are combined together to thereby change the combined focal length of the objective lens, but in the low magnification state, an additional lens is inserted rearwardly of said another negative lens and at least two negative lenses are used as the objective lens and therefore, such system has the tendency that the full length becomes greater if an attempt is made to enhance the negative refractive power of the additional lens and change the magnification change ratio, and this is more disadvantageous for providing a compact viewfinder as the magnification change ratio becomes higher.
Also, in the viewfinder of Japanese Laid-Open Patent Application No. 166933/1985, which is of the type which assumes a two-group construction of a negative lens and a positive lens in the low magnification state and assumes a three-group construction of a positive lens, a negative lens and a positive lens with a positive lens added to the two-group construction, the effective diameter of the positive lens as a first lens added in the high magnification state becomes great with a result that the viewfinder becomes bulky. Also, this viewfinder is designed such that for a magnification change of the viewfinder, the negative lens is moved and the positive lens is additionally inserted in place of it, and this leads to the disadvantage that a moving mechanism for the lens becomes complicated.
The viewfinder of Japanese Laid-Open Patent Application No. 26323/1985, like the present invention, is of the type in which the negative lens as the objective lens is interchanged to the thereby change over the viewfinder magnification. However, its magnification change ratio is up to the order of 1.78 times as shown in the embodiment of this patent application. To further increase this magnification change ratio so as to correspond to the magnification change ratio of a photo-taking lens, it is necessary to weaken the refractive power of a negative lens for high magnification or strengthen the refractive power of a negative lens for low magnification. However, if the refractive power of the negative lens for high magnification is weakened, that negative lens comes close to an optical member on the eyepiece side and the marginal portion of the second surface directing a concave surface toward the eyepiece side may contact the optical member on the eyepiece side and therefore, the magnification cannot be made sufficiently high. Also, if the refractive power of the negative lens for low magnification is strengthened, there is the disadvantage that the viewfinder magnification in the low magnification state becomes too small.